Communication methods and devices have traditionally relied on audiovisual modes to convey the message from a source to a recipient. Audiovisual modes are capable of conveying considerable amounts of information within a reasonable time period with acceptable accuracy. The primary audiovisual modes of communication have relied on the receiver's eyes and ears.
A lesser known and relatively uncommon mode of communication is tactile communication. As discussed below in more detail, efforts to develop this mode of communication have been limited and typically geared towards improving the communication reception for people disabled or impaired in either hearing or vision. Tactile communications has not found use in the general population essentially because of the overwhelming reliance on audiovisual modes of communication.
Development of communication devices using the sense of touch in general have suffered because of a general lack of knowledge in the area of understanding the sense of touch. There is also a relative lack of sophistication of the sense of touch when compared with the acuity found for the senses of hearing and vision. By comparison, the sense of touch exhibits difficulty with localization and perception of a stimulus. Humans are able to see extremely fine patterns of surface asperity that nonetheless feels as smooth as glass. The relatively low level of sophistication of the sense of touch remains somewhat baffling in light of the myriad types of end organ receptors that provide the brain with tactile information about our environment obtained through the sense of touch.
Within the human body, tactile conditions are monitored through an interaction of neuron end organ receptors within the skin and internal organs and musculoskeletal system of the human body. Stimulation of tactile end organs sends a stimulus along the neuron of that end organ to the sensory cortex of the human relaying the information typically carried by these nerves. There are a number of different types of end organ effectors within a human. The general areas of sensory modality detected by skin receptors fall in the categories of fine or light touch, coarse touch, vibratory, pressure, pain, heat, and cold. Mechanoreceptor end organs are present within the human body in muscles, tendons and joints and provide important information concerning musculoskeletal positioning and movement. Consequently, the sense of touch can be distinguished by dividing into two general categories, the first being the exteroceptive sensory modality and the second category is the proprioceptive sensory modality. The general subject of this present invention will be concerned primarily with the exteroceptive tactile sensory modality.
The neuron end organ effectors in the exteroceptive modality are a diverse assortment of organelles. There are free nerve endings, Merkel's discs, Meissner's corpuscles, Pacinian corpuscles, and Ruffini's endings supplying tactile sensation to the skin. Free nerve endings predominate and are found generally throughout the entire skin surface area. Free nerve endings typically innervate the layers of the skin as unmyelinated fibers carrying primarily pain as well as hot, cold and light touch. Free nerve endings with medium myelinated fibers are associated with hair follicles within the skin and predominantly supply light touch sensations.
Meissner's corpuscles are predominantly associated with the thicker skin of the palms and fingertips of a hand and the sole and toe tips of the feet and primarily provide the perception of light touch in these areas. The high density of Meissner's corpuscles in the hands and feet is the underlying anatomic basis behind the relatively exquisitely sensitive tactile abilities associated with the hands and feet and two point discrimination. In contrast, Meissner's corpuscles are rare elsewhere in the thinner skin of the human body. Contrast the decrease in Meissner's corpuscles with a relative increase in the number of hair follicles and associated free nerve ending fibers that provide the light touch sensory modality to those areas of skin not associated with the palms or soles of the body. Consequently, the differences in light touch between the palms, soles and the skin of the rest of the body lies not only in the difference in neuron end organ effectors but also in whether the nerve fiber is myelinated or non-myelinated. Merkel's corpuscles predominantly give rise to vibratory sensing ability. As a consequence, Merkel's corpuscles have less sensitivity to location and two point discrimination but exquisitely sensitive to spatial resolution of complex surface patterns when the fingers are scanned over an object or the object moved over the fingers.
The quality of sensory ability is dependent on the ability of an end organ effector (or free nerve ending) to sense the presence of a stimulus, respond to that stimulus by propagating a signal along the length of the neuron, recharge the neuron after its firing, and regain sensitivity to a stimulus following reception of the previous stimulus. These general areas of qualification of nerve function are threshold, conduction velocity, refraction, and adaptation. Non-myelinated fibers are generally slower to conduct, have higher periods of refractoriness and quickly adapt to external stimuli relative to myelinated fibers and the converse is true wherein the greater the degree of myelination the higher the conduction velocity, the shorter the period of refractoriness and the less susceptible to adaptation the nerve becomes. Additionally, the more sophisticated neuron end organs such as Pacinian corpuscle, Merkel's corpuscle, Meissner's corpuscle, and Ruffini's corpuscle generally share a higher degree of sophistication as to structure and are associated with medium myelinated fibers. Contrast this with heavily myelinated fibers used in the proprioceptive sensory modality where position sense, muscle force contraction and joint position are relatively refined and sophisticated allowing us to perform fairly complex fine motor athletic movement. The heavily myelinated fibers having the highest rate of conduction, the shortest period of refractoriness and the greatest resistance to adaptability.
Threshold of a nerve fiber will depend in great part to the type of neuron end organ effector present on that nerve fiber. The threshold of firing will also depend on the type of stimulus being presented to the neuron end organ. Free nerve endings along the basement membrane of the cutaneous layer of the skin have little, if any, end organ structure to them and have fairly low thresholds for firing. Free nerve endings are also found to be fairly diffuse with the free end organs branching a number of times and innervating a substantial area of skin in proportion to the size of the nerve fiber supplying that area. Consequently, the quality of signal received from free nerve endings has a generally diffuse character poorly localized when compared to light touch provided by a Meissner corpuscle. A Meissner corpuscle is arranged in a tiered fashion of epithelial cells within the corpuscle with the main axis of the corpuscle perpendicular to the surface of the skin. This tiered arrangement, much like a stack of pancakes where each pancake represents a specialized epithelial cell and a nerve ending between the pancakes, is oriented in such a way as to be very sensitive to slight pressures applied along its major axis and relatively insensitive to pressures arriving from a lateral direction. This directionality of a Meissner corpuscle contributes to its greater ability to finally localize and discriminate from two different points accurately. Contrast a Meissner corpuscle with a Pacinian corpuscle which essentially is a laminated body surrounding a single nerve ending. This lamellar construction with the nerve ending at its center provides increased sensibility to pressure from all directions but because of a lack of orientation there is less sensitivity to discriminate size and location of the pressure stimulus.
Conduction velocity is a measure of the speed with which a nerve will transmit to the sensory cortex of the brain the fact that a stimulus has arrived at the nerve end organ. Myelination provides for higher conduction velocities where more myelin is associated with faster conduction velocities.
When a nerve threshold is reached, the nerve fires and conducts a signal stimulus along its length and must then recharge the nerve in order to be ready to respond to the next stimulus. The length of time that the nerve is discharged is known as the refractory period. The refractory period is a state of non-responsiveness on the part of the nerve in that it cannot respond to a continuing external stimulus during this period.
Neuronal adaptation is that ability of the nerve to modify its level of sensitivity to changes in the environment. In effect, the neuron becomes used to the external stimulus and reestablishes a new level of response to stimuli.
Tactile communication not only relies on the ability to sense that a touch has in fact occurred but also determine the nature of the touch. The touch should convey useful information. An example might be placing a car key in the hands of a blindfolded subject. The subject should be able to tell you not only that their hand has in fact been touched but be able to discern from the pattern of the stimulus that you have placed a car key in their hand. This level of perception is defined as stereognosis which is the appreciation of a form of an object by means of touch. With the perception of a key, the subject is able to tactually feel a continuous surface and edge. In light of the anatomy and distribution of neuron end organs contributing to tactual perception, the spatial resolution is limited by the spacing of single nerve fibers in the immediately surrounding area adjacent to that single nerve fiber and its end organ. As in the case of the fingertip and the high density of Meissner's corpuscles, the perception of a spatial form on the skin of the fingertip would depend on a neuronal image of the stimulus established by the density of the Meissner corpuscles. The greater the density, the greater the perceptual ability to perceive complexity and the greater the spatial resolution. The effect of this density pattern of neuron end organs becomes readily apparent when considering a subject's ability to discriminate between two points. Our tactile ability to resolve a form spatially is enhanced if we then rub our fingers over the object, such as the key. This scanning motion sets up a vibratory sensation to which Merkel's corpuscles may respond. The vibratory sensation builds up an image that is resolvable at dimensions less than a millimeter.
In the article titled "The Perception of Two Points is not the Spatial Resolution Threshold", K. O. Johnson et al., in Touch, Temperature, Pain and Health Diseases: Mechanisms and Assessments, Progress and Pain Research and Management, Vol. 3, edited by J. Boivie et al., the authors discuss the tactual difference in perceiving a two point discrimination versus spatial pattern recognition thresholds. In their review, Johnson et al. discuss the responses evoked by one and two point stimuli versus the neural mechanisms associated with tactile spatial resolution. The results demonstrate that there is a distinctly different mechanism of response by a human subject when presented with a single probe, a double probe or a more complex vibratory pattern. Furthermore, they are able to show that response to one and two point stimuli will produce different sensations depending upon longitudinal or transverse orientations of the probes which would allow discrimination between one and two points stimuli on the basis of cues that may have had nothing to do with spatial resolution. Furthermore, they were able to demonstrate that the threshold of tactile spatial resolution has remained independent of the two point discrimination threshold. It has been shown that the neurologic system responsible for tactile spatial pattern recognition at the limits of resolution is the slowly adapting type 1 (SAI) afferent fiber system. The individual SAI afferent fibers terminate in Merkel receptors and have high spatial resolving capacity. Contrast this with rapidly adapting (RA) afferents which terminate in Meissner corpuscles and have poor spatial resolving properties. Meissner corpuscles have relatively high density in the fingertips and palms of the hand and toes and sole of the feet. As noted above, this high density provides for substantially increased two point discrimination resolution, i.e., the ability of a subject to determine whether they are being touched by a single or two separate probes simultaneously. Contrast this with a plurality of probes that are in a spatially configured pattern, for instance the letter "A", such that if all of the probes come into contact with the skin surface of a fingertip simultaneously, the question becomes will the subject be able to discern and resolve the spatial configuration of the multiple probes if the probes are spaced together less than the two point discrimination threshold or if the probes are spaced apart greater than the two point discrimination threshold.
To evaluate this question, consider a device known as the Optacon developed by Bliss and noted in the paper "Summary of Optacon Related Cutaneous Experiment". In the conference on cutaneous communication systems and devices, F. A. Geldard, editor of the Psychonomic Society, 1974. The Optacon uses an array of 144 probes in a 12.times.12 pattern. The array measures approximately one to one and one-half centimeters on each side. Consequently, the distance between one probe and its nearest partner is approximately one millimeter. The Optacon takes visual representation of a letter or number as its input and extends the appropriate number of probes from the surface of the array to spatially correspond to the letter or number being visualized. For example, the letter "A" may use upwards of thirty probes simultaneously contacting the skin of a subject's fingertip with each probe no greater than approximately one millimeter from its nearest neighbor. Therefore, if the two point discrimination threshold is two millimeters, all thirty of the protruding probes from the array will be indistinguishable from each other and perceived as a single probe fairly broad in its size.
As demonstrated and discussed above, Meissner corpuscles are predominantly responsible for two point discrimination. Merkel discs, by contrast, are responsible for spatial resolution. However, to take advantage of Merkel's disc stimulation, the Optacon and similar devices such as U.S. Pat. No. 3,229,387 issued Jan. 18, 1966 to Linvill, use a plurality of probes in a fairly large array such as the 12.times.12 array of the Optacon. The array is used to scan across a page of letters and numbers while attached to a fingertip surface and the letters and numbers through protruding vibrating probes are then felt to scan across the fingertip much as a ticker tape output scans across a marquee. For example, the Optacon slides across the letter "A" and the letter "A" is felt to slide across the fingertip of the wearer of the Optacon. Sequential numbers of probes in the pattern "A" protrude from the surface of the array and vibrate at a set frequency. Depending upon the size of the letter there may be upwards of thirty or forty probes simultaneously vibrating against the surface of the subject's fingertip. It is the combination of the changing sequence of simultaneously vibrating probes and the vibration of the probes that contributes to the subject perceptually identifying the spatial resolution of the letter. If the letter were to not scan but remain static with the thirty or so probes arranged in a letter but vibrating against the subject's fingertip, the subject would not resolve the pattern into any useful recognizable alphanumeric. And as noted above, it has been shown that the SAI fibers terminating in Merkel's discs contribute to the spatial resolution perceived by a subject using a device similar to the Optacon.
Tactile stimulators may be generally divided into two groups: The synthetic systems and the analogic systems. These systems are devices in which the cutaneous sensory system is intended to replace one of the other sensory systems, most commonly vision or hearing. Examples of analogic audio systems are cochlear implants that convert sounds such as speech into tactile sensations felt by a subject at a site designed to be used by the device. The ability to transmit speech to the skin using a single vibrating transducer generally has failed in attempts. Continued work in this area has led to the development of systems which electrically divide the speech spectrum into different frequency bands. These various bands may also be modified in terms of time delay schemes and positioning to more closely accommodate the direction of the actual sound source.
Other audio tactile aids are known as vocoders. A number of vocoder devices have been tested and an evaluation of two multichannel tactile aids can be found in the paper "Evaluation of Two Multichannel Tactile Aids for the Hearing Impaired", Weisenberger et al. in the Journal of Acoustical Society of America, Vol. 86 (5), pp. 1764-1775, November 1989. The two vocoder devices described used 16 element linear vibratory arrays displaying activity in 16 overlapping frequency channels. The 16 elements vibrated simultaneously with the frequency ranges approximately a third of an octave in bandwidth spaced evening over the frequency range between 140 to 6,350 Hz. Accuracy of communicating with these vocoders was limited with subjects being able to identify only 70% of a 250 word test list even when combined with lip-reading.
All of these analogic systems replacing hearing use multiple vibrotactile probes vibrating simultaneously at frequencies approximating those of actual speech. These systems have proven difficult to incorporate and accurately rely on.
Visual analogic systems are represented by such devices as the Optacon or a tactile vision information system (TVIS) as described in "Effective Tactile Stimulation Pulse Characteristics on Sensation Threshold and Power Consumption", Nuziata et al., Annals of Biomedical Engineering, Vol. 17, pp. 423-35, 1989. The authors describe the basic function of the TVIS as the acquisition of an optical image with a video camera and the transformation of the image or some portion of the image into a vibratory pattern on a specific region of skin. Like the Optacon, the TVIS uses a vibratory tactile array coupled with appropriate electronic frequency filtering in order to create a spatial analog of the visual scene being picked up by the video camera. Each vibrator used a base frequency of 250 Hz. The choice of 250 Hz was dependent on the minimum threshold for tactile sensation using Pacinian corpuscles that are the most responsive end organ receptors in the vicinity of 250 Hz stimulation frequencies. Both the Optacon and the TVIS use multiple vibrating probes in a spatial pattern to create the vibrotactile message discerned by the subject using the device.
While an analogic system such as the Optacon, where the device uses multiple simultaneous vibrotactile probes to create a complex spatial form, confusion and difficulty with perception has been studied. In the paper "The Effects of Complexity on the Perception of Vibrotactile Patterns", Homer, Perception and Psychophysics, line 49 (6), pp. 51-62, 1991, the author identified tactile confusion for letters with a greater number of lines such as the letters M, W, B and K. Therefore, despite the spatial threshold being significantly less than the two point discrimination threshold, the difference of 0.9 millimeters versus approximately two millimeters for the overlaying skin of a fingertip, complexity of the spatial form remains an obstacle difficult to overcome.
Synthetic systems employ communicating with languages employing synthetic codes. Braille is the most useful, best known and longest lived example of the synthetic families of tactile codes. Braille uses a 2.times.3 array to form unique patterns discernible as the alphabet. Consequently, the tactile experience does not resemble either the visual or auditory experience associated with the letter for which the pattern stands. In the simplest of terms, synthetic systems require that the user of the system learn the additional language set employed by the synthetic system.
Translation of Braille into a vibrotactile device would necessarily require an array 2.times.3 and be capable of simultaneously vibrating up to all six of the probes. As with the Optacon, to achieve the smaller sizes and utilize the lower threshold associated with spatial resolution of complex forms, the Braille patterns would necessarily need to be scanned across the skin surface, preferably the tip of a finger. Consequently, a device useful for tactually displaying Braille figures would need an array having substantially greater than six vibromechanical probes. Without the scanning capability, a device incorporating Braille as the underlying interpretive language would use a minimum of six vibromechanical probes, each requiring spacing between probes to be greater than the two point discrimination threshold. This minimum spacing is necessary to allow the subject wearing the device to discern between two or more probes, since Braille characters require anywhere from one to six simultaneously vibrating probes.
The communication systems described above have been developed as devices to provide communication devices to subjects who are otherwise impaired with either visual or auditory abilities to communicate. Whether synthetic or analogic, these systems generally rely on Merkel's discs densely populating the fingertips to achieve spatial resolution thresholds low enough to communicate complex spatial forms such as letters and numbers.
There exist other tactile phenomena that are not well understood. An example of such a tactile phenomenon is described in the paper "Apparent Haptic Movement" by Sherrick, et al., in Perceptions and Psychophysics, Vol. 1, pp. 175-180, 1966, wherein the author describes the induction of a sense of movement produced by stationary vibrators sequentially fired over the surface of the subject's body. The authors describe one example where an intense sense of rotational motion was induced by successively firing six vibrators placed around a subject's chest. The authors further studied a subject's sensation of haptokinetic movement employing a device with two vibrators spaced at different distances, from 4 to 22 centimeters, along the length of the subject's leg. The subject was allowed to control the duration of the two vibrotactile bursts as well as the interval of time between the onset of the two vibrotactile bursts. In this way, the subject was able to adjust the sequential firing of the two vibrators to achieve a maximum perception of haptic movement between the two vibrators. For each trial run, the vibrators were vibrated at 150 Hz for burst durations ranging from 25 to 400 milliseconds (msec) which equates to from 4 to 60 vibrations per burst. The interval between burst onsets ranged from 75 to 400 msec.
A different tactile phenomenon was induced in subjects using a system slightly different than the previously described system as outlined in the journal article "The Cutaneous `Rabbit`: A Perceptual Illusion" by Geldard et al. in Science, Vol. 176, pp. 178-179, Oct. 13, 1972. These authors used from two to five vibrators consisting of a short length of lucite rod about 0.6 centimeters in diameter with a rounded tip rigidly mounted on Clevite bimorph benders and driven by a pulse generator generating a square wave pulse 2 msec in duration. Each vibrator received five pulses separated anywhere from 40 to 80 msec between each pulse. The vibrators were aligned in a linear array over a subject's forearm and upper arm on an average spacing of approximately ten centimeters with a range from two centimeters to 35 centimeters. The phenomenon experienced by the subjects in the test was the sensation of a smooth progression of jumps, or taps, on the arm between the successively firing vibrators. It was described as if a tiny rabbit were hopping from one vibrator to the next. If the number of vibrator taps is increased for each vibrator then the hops become shorter and closer together and the opposite effect is also noted. The authors distinguished this rabbit effect from the vibrotactile movement described above by Sherrick, et al., on the basis that the rabbit effect gives a discontinuous hopping sensation described as discreet taps between the stimulus loci which is in contradistinction to the continuous vibrating gouging sensation in the skin between loci experiencing the vibrotactile or haptokinetic movement illusion.
The devices that attempt to replace vision or hearing do so by relying on a plurality of vibrators firing simultaneously to reproduce either a complex spatial arrangement such as a letter or number or to recreate the vibrations associated with speech. These systems consume a considerable amount of power to fire the plurality of vibrator arrays simultaneously and are dependent on their interaction with Merkel's discs, Meissner's corpuscles or Pacinian corpuscles to relay the communication information from the vibromechanical device to the conscious awareness of the recipient.
The perceptual phenomena described with the vibrotactile or haptokinetic movement and the rabbit affect appear to be independent of Meissner's corpuscles, Merkel's discs or Pacinian corpuscles since the vibromechanical stimulators are placed independent of, and in fact can be varied in their distance between each locus and still create the illusion of movement between the stimulator loci. These phenomena appear to be more a function of perception at the sensory cortex level as the stimuli are reconstructed in real time and perceived at a conscious awareness level by the subject. Therefore, these phenomena appear to be independent of two point discrimination and spatial resolution thresholds.
There does not as yet exist a vibromechanical tactile communication device capable of universal use that can receive and convey information to the wearer conveniently or accurately.